Clarification of Counter.increment/decrement with negative values

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Javadoc for methods now clarifies what happens when negative values for 
Counter.increase/decrease are used:
- they will affect increment/decrementSum respectively, making it lower
- they will adjust min/max as expected (with their timestamps):
-- after increase(1) max is set
-- after increase(-1) min is set as well
-- similarly for decrease

Original issue reported on code.google.com by virgo47 on 16 Mar 2014 at 11:17